Heretofore, an alkali-free glass containing no alkali metal oxide has been used as the glass plate for an LCD panel. This is because when an alkali metal oxide is contained in the glass substrate, during a heat treatment performed in the production process of an LCD panel, an alkali ion in the glass substrate may diffuse into a semiconductor film of a thin-film transistor (TFT) used for driving an LCD panel and cause deterioration of the TFT characteristics.
Also, by virtue of having a low thermal expansion coefficient and a high glass transition temperature (Tg), the alkali-free glass is less likely to undergo a dimensional change in the production process of an LCD panel and has small effect on the displaying quality due to thermal stress during use of the LCD panel and therefore, is preferred as the glass substrate for an LCD panel.
However, the alkali-free glass has the following drawbacks in view of production.
The alkali-free glass is very highly viscous and has a property of being difficult to melt, and its production involves technical difficulty.
Furthermore, in general, a refining agent is poorly effective for an alkali-free glass. For example, in the case of using SO3 as the refining agent, since the temperature at which SO3 is (decomposed and) released as bubbles is lower than the glass melting temperature, the most part of SO3 added is decomposed and volatilized from the molten glass before fining is achieved, and the fining effect cannot be sufficiently exerted.
Use of an alkali metal oxide-containing alkali glass substrate as the glass substrate for a TFT panel (“a-Si for TFT panel”) has been also proposed (see, Patent Documents 1 and 2). This is because the heat treatment in the production process of a TFT panel becomes possible to be performed at a relatively low temperature (approximately from 250 to 300° C.), while it has been conventionally performed at 350 to 450° C.
The alkali metal oxide-containing glass generally has a high thermal expansion coefficient and therefore, in order to obtain a thermal expansion coefficient preferred as the glass substrate for a TFT panel, B2O3 having an effect of reducing the thermal expansion coefficient is usually incorporated (see, Patent Documents 1 and 2).
However, in the case where a glass composition containing B2O3 is formulated, B2O3 is volatilized upon melting of the glass particularly in a melting step, a fining step and a float forming step, and in turn, the glass composition is liable to become inhomogeneous. When the glass composition becomes inhomogeneous, this affects the flatness in forming the glass into a plate shape. In order to secure the displaying quality, high flatness is required of the glass substrate for a TFT panel so that the distance between two sheets of glass sandwiching a liquid crystal, i.e., the cell gap, can be kept constant. Therefore, after forming a sheet glass by a float process, the surface of the sheet glass is polished so as to secure a predetermined flatness, but when the sheet glass after forming does not have a predetermined flatness, the polishing step requires a long time and the productivity is reduced. Also, considering the environmental burden caused by volatilization of B2O3, lower content of B2O3 in the molten glass is preferable.
However, when the B2O3 content is low, it has been difficult to reduce the thermal expansion coefficient to a level preferred as a glass substrate for a TFT panel and achieve a predetermined Tg or the like while suppressing the increase in viscosity.